President Barack Obama has laid into his would-be White House replacement in a fervent attack on Mitt Romney's policies and character designed to knock his challenger off course ahead of a crucial convention speech.

In a calculated move timed just days before Republican delegates meet in Florida to nod through Romney as their presidential candidate, Obama accused his rival of holding "extreme positions" and failing to "own up" to responsibilities.

In an interview with the Associated Press, the president said that contrary to common belief, he had no beef with the former Massachusetts governor on a personal level, adding: "I don't really know him well."

But he attacked Romney on positions the candidate has taken that tie him to the right wing of his party, and cast aspersion over whether the Republican presidential hope had adopted stances out of conviction or through political expediency.

In the interview conducted Thursday but released on Saturday, Obama said: "I can't speak to governor Romney's motivations," adding: "What I can say is that he has signed up for positions, extreme positions, that are very consistent with positions that a number of House Republicans have taken.

"And whether he actually believes in those or not, I have no doubt that he would carry forward some of the things that he's talked about."

The president cited Romney's insistence on tax relief for America' super-rich as evidence of his "extreme" positions.

He also sought to tie the Republican presidential challenger to the recent furore surrounding conservative a Senate candidate, Missouri's Todd Akin.

Akin has been roundly condemned for comments he made suggesting that "legitimate rape" rarely leads to pregnancy.

Romney himself has described the remarks as "inexcusable" and led party calls for him to step down as a Senate candidate. And he is on record as not opposing abortion in cases of rape and incest or if it will save the mother's life.

But Obama noted in the Associated Press interview that calls for a "no exceptions rule" in regards to a ban on abortion – the position Akin was ham-fistedly trying to defend – are part of the Republican platform in Tampa

The White House incumbent also predicted that a President Romney would not "stand in the way" if Congress gave him a bill that stripped away women's control over their reproductive health.

The comments reflect a belief in Democratic circles that conservative positions adopted by Republicans on social issues could alienate the party from moderate and independent voters.

The president also aimed a blow at Romney's personal tax arrangements – another common area of attack in recent months.

Romney refusal to release more tax returns for public inspection displayed a "lack of willingness to take responsibility for what this job entails".

As to his own record, Obama agreed that in regards to the economy, America was not "where we need to be".

But he added: "Governor Romney's policies would make things worse for middle-class families and offer no prospect for long-term opportunity for those striving to get into the middle class."

The timing of the interview – just 10 weeks out from the general election and days before the Republican convention is due to begin – is clearly designed to steal a march on Romney ahead of his big week in Florida.

A GfK poll released on Saturday suggest that the White House race is still on a knife-edge, with 47% of respondent saying they plan to back Obama and 46% opting for Romney.

But despite the even split in voter intention, most people still expect Obama to win. The GfK survey found that 58% believe the Democrat will remain in power, with just 32% saying they thought the White House keys would exchange hands in November.

In the Associated Press interview, Obama expressed confidence that voters would stick with him in the general election.

"If they saw governor Romney offering serious proposals that offered some sort of concrete ways in which middle-class families would be helped, then I could understand them thinking about that choice," Obama said.